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  <h1 id="深入理解javascript的闭包"><a href="#深入理解javascript的闭包" class="headerlink" title="深入理解javascript的闭包"></a>深入理解javascript的闭包</h1><a id="more"></a>
<h2 id="首先回顾一下作用域链"><a href="#首先回顾一下作用域链" class="headerlink" title="首先回顾一下作用域链"></a>首先回顾一下作用域链</h2><p>直接上来举个简单的栗子 </p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//window</span></span><br><span class="line"><span class="keyword">var</span> a = <span class="number">1</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">aa</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">	<span class="keyword">var</span> b = <span class="number">2</span></span><br><span class="line">	<span class="function"><span class="keyword">function</span> <span class="title">bb</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">		<span class="keyword">var</span> c = <span class="number">3</span></span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这里涉及三个执行环境：全局，函数aa，函数bb。<br>1、全局环境：可访问变量a与函数aa。全局变量对象为<strong>{a : 1, aa : (function)}</strong><br>2、函数aa：可访问变量a，变量b，函数bb。aa变量对象为<strong>{b : 2, bb : (function), arguments}  {a : 1, aa : (function)}</strong><br>3、函数bb：可访问变量a，b，c。bb变量对象为<strong>{c : 3, arguments}  {b : 2, bb : (function), arguments}     {a : 1, aa : (function)}</strong></p>
<p>简单来说，作用域就是变量与函数的可访问范围，即作用域控制着变量与函数的可见性和生命周期</p>
<p>红宝书上的概念：当代码在一个环境中执行时，会创建变量对象的一个作用域链。他的作用是保证对执行环境有权访问的所有变量和函数的有序访问。在作用域链中只能向上访问而不能向下访问。<br>若环境是函数，则将其<strong>活动对象</strong>作为变量对象，活动对象一开始只包含一个arguments对象。其下一个变量对象来自包含环境，再下一个变量对象来自下一个包含环境，这样一直持续到全局环境，形成一个作用域链。</p>
<h3 id="延长作用域链"><a href="#延长作用域链" class="headerlink" title="延长作用域链"></a>延长作用域链</h3><p>由于有些语句可以在作用域链<strong>前端</strong>临时增加一个变量对象，该变量对象会在代码执行之后被移除<br>1、try-catch语句的catch块<br>2、with语句<br><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">a</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">	<span class="keyword">with</span>(location)&#123;</span><br><span class="line">		<span class="keyword">var</span> url = href + <span class="string">'?name=Tom'</span></span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">return</span> url</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure><br>有了这个语句，函数a这个执行环境中的变量对象就包含了location对象及其所有属性，相当于把location对象挂在了作用域链的最前端。with语句中的href实际上是location中的href。而在with内部定义的url变量，也会曾为函数变量对象的一部分</p>
<h2 id="闭包"><a href="#闭包" class="headerlink" title="闭包"></a>闭包</h2><p>创建闭包最常用的方式，就是在一个函数内部创建另一个函数。简单来说：函数 A 返回了一个函数 B，并且函数 B 中使用了函数 A 的变量，函数 B 就被称为闭包。<br>首先直接来个栗子：<br><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">A</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="keyword">let</span> a = <span class="number">1</span></span><br><span class="line">  <span class="function"><span class="keyword">function</span> <span class="title">B</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">      <span class="built_in">console</span>.log(a)</span><br><span class="line">  &#125;</span><br><span class="line">  <span class="keyword">return</span> B</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> C = A()</span><br><span class="line">C() <span class="comment">// 1</span></span><br></pre></td></tr></table></figure><br>在上述例子中，函数A的活动对象为{arguments, a : 1, B : (function)}，当A()执行之后函数A即被销毁，但是由于B使用到了A的活动对象内容，因此A的活动对象会被保存到内存。函数C执行可以返回正确的内容</p>
<p>但在这种模式下，就会产生js闭包的一个经典问题（闭包循环返回最后一个值的问题），红宝书中这样描述：闭包只能取得包含函数中任何变量的最后一个值。闭包所保存的是整个变量对象，而不是某个特殊的值。<br>红宝书上的两个例子：<br><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//例子1</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">a</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">	<span class="keyword">var</span> res = [];</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">var</span> i = <span class="number">0</span>; i &lt; <span class="number">10</span>; i++)&#123;</span><br><span class="line">		res[i] = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">			<span class="keyword">return</span> i</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">return</span> res</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> c = a()</span><br><span class="line">c[<span class="number">0</span>]() <span class="comment">//10</span></span><br><span class="line">c[<span class="number">1</span>]()<span class="comment">//10</span></span><br><span class="line">...</span><br><span class="line"></span><br><span class="line"><span class="comment">//例子2</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">a</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">	<span class="keyword">var</span> res = [];</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">var</span> i = <span class="number">0</span>; i &lt; <span class="number">10</span>; i++)&#123;</span><br><span class="line">		res[i] = (<span class="function"><span class="keyword">function</span>(<span class="params">num</span>)</span>&#123;</span><br><span class="line">			<span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">				<span class="keyword">return</span> num<span class="comment">//这里保存了不同的num副本</span></span><br><span class="line">			&#125;</span><br><span class="line">		&#125;)(i)</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">return</span> res;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> c = a()</span><br><span class="line">c[<span class="number">0</span>]<span class="comment">//0</span></span><br><span class="line">c[i]]<span class="comment">//1</span></span><br><span class="line">...</span><br></pre></td></tr></table></figure><br>上述两个例子中，区别在于，res[i]=的后面，例子一是直接将闭包赋值给了res，例子二是又创建了一个闭包函数赋值给res。</p>
<p>前者不立即执行，而是当res返回之后手动调用res才执行，这个时候函数a中循环结束，变量对象中保存有i的值为10，所以在之后调用res函数时，无论调用res数组的哪个函数，只会返回10</p>
<p>后者由于是一个立即执行的匿名函数，因此生成的<strong>10个匿名函数</strong>都会正确得到 i 的值，然后再将这些值依次通过num<strong>值传递</strong>给内部的闭包函数，并将闭包函数返回。</p>
<p>总结来说，前者10个函数共享同一个变量对象，<strong>这个变量对象在res返回之后才被保存使用</strong>，因此值为10，；后者10个函数分别使用不同的父级环境的变量对象，<strong>这些变量对象是随着for循环依次保存给内部闭包使用</strong>，因此内部闭包保留有不同的num副本，可以返回正确的值</p>
<p>更多例子：<a href="https://blog.csdn.net/weixin_43586120/article/details/89456183" target="_blank" rel="noopener">闭包，看这一篇就够了——带你看透闭包的本质，百发百中</a><br>（转载，侵删）<br>其中，个人认为这个例子能最简单地看清闭包的本质。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">m1</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">     <span class="keyword">var</span> x = <span class="number">1</span>;</span><br><span class="line">     <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">          <span class="built_in">console</span>.log(++x);</span><br><span class="line">     &#125;</span><br><span class="line">&#125;</span><br><span class="line"> </span><br><span class="line">m1()();   <span class="comment">//2</span></span><br><span class="line">m1()();   <span class="comment">//2</span></span><br><span class="line">m1()();   <span class="comment">//2</span></span><br><span class="line"><span class="comment">//使用不同的变量对象</span></span><br><span class="line"><span class="keyword">var</span> m2 = m1();</span><br><span class="line">m2();   <span class="comment">//2</span></span><br><span class="line">m2();   <span class="comment">//3</span></span><br><span class="line">m2();   <span class="comment">//4</span></span><br><span class="line"><span class="comment">//引用同一个变量对象</span></span><br></pre></td></tr></table></figure>
<p><strong>总结：就是认清楚到底是什么时候保存的被销毁函数的变量对象</strong><br>当然，还有一个小坑值得一提</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Foo</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">var</span> i = <span class="number">0</span>;</span><br><span class="line">    <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(i++);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"> </span><br><span class="line"><span class="keyword">var</span> f1 = Foo(),</span><br><span class="line">    f2 = Foo();</span><br><span class="line">f1();</span><br><span class="line">f1();</span><br><span class="line">f2();</span><br></pre></td></tr></table></figure>
<p>这段代码的输出是0，1,  0<br>我一开始认为f1和f2都=foo()是都指向了同一个function引用类型，所以顺理成章就会答错认为：0 1 2<br>但其实foo()返回的是一个匿名函数，所以f1,f2相当于指向了两个不同的函数对象，所以结果也就是0 1 0</p>
<h2 id="闭包带来的问题：内存泄漏"><a href="#闭包带来的问题：内存泄漏" class="headerlink" title="闭包带来的问题：内存泄漏"></a>闭包带来的问题：内存泄漏</h2><p>合理使用闭包可以很好的实现封装与缓存<br>但是需要注意的是，闭包的特性使得其一些变量与参数不会被js的垃圾回收机制所回收，所以在老版本的IE中，如果使用闭包不当，会造成消耗内存和内存溢出的问题。<br>举个栗子：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">a</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">	<span class="keyword">var</span> element = <span class="built_in">document</span>.getElementById(<span class="string">'test'</span>)</span><br><span class="line">	element.onclick = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">		<span class="built_in">console</span>.log(element.id)</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">//以上函数由于保存了函数a的活动对象的引用，因此element占用的内存将永远不会被释放</span></span><br></pre></td></tr></table></figure>
<p>这个问题可以用以下方法暂时解决</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">a</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">	<span class="keyword">var</span> element = <span class="built_in">document</span>.getElementById(<span class="string">'test'</span>)</span><br><span class="line">	<span class="keyword">var</span> id = element.id</span><br><span class="line">	element.onclick = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">		<span class="built_in">console</span>.log(id)</span><br><span class="line">	&#125;</span><br><span class="line">	element = <span class="literal">null</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>上述代码中，我们将element的id值保留着在一个副本中，防止闭包函数对element的直接引用，但一定要记住的是，由于闭包保存的是执行环境的整个活动对象，所以即使没有直接引用element，活动对象{element, id, …}中element依然存在，因此需要将element设置为null以<strong>回收内存</strong>，否则并不会解决内存泄漏的问题</p>
 
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